1. Field of the Invention
The invention relates to ultrafiltration membranes. More particularly, it relates to the production of dry, crack-free membranes having enhanced stability.
2. Description of the Prior Art
The production and use of inorganic, semipermeable membranes for ultrafiltration purposes are well known in the art. Most such inorganic membranes are of advantage in their resistance to temperature and solvent effects. In some instances, the membranes also possess molecular perm-selectivity and ion-exchange properties. Berger, U.S. 3,497,394 thus disclosed an ion exchange membrane made by forcing a metal oxide gel into a porous support.
In the practical application of ultrafiltration membranes, high flux is an essential feature, and it has been found desirable to have a highly porous support and a thin, fine membrane. Colloidal particles thus should not be imbedded in depth into the filter body. In a dry, inorganic, semipermeable filter disclosed in French Pat. No. 1,440,105, however, fine colloidal particles are said to create a thin membrane in the surface of a porous combined membrane--support formed from a suspension of coarse and colloidal size particles of .gamma.-Al.sub.2 O.sub.3 by slip casting in a plaster mold.
Ultrafiltration membranes should also have good mechanical and chemical stability for use in practical commercial applications. Dehydration or ordinary particulate membranes always result, however, in "mud cracks" that ruin the membranes' semipermeable characteristics. In the preparation of a support surface, the formation of cracks can of course, be tolerated. In the Thomas patent, U.S. Pat. No. 3,926,799, for example, a membrane support is made by coating a zirconia slurry onto a porous substrate, followed by drying and firing the resulting composite at high temperature to form a rugged precoat. Large pores or cracks would be expected to form in this process and would be unacceptable in the formation of the membrane itself. Ultrafiltration membranes susceptible to such crack formation must be maintained wet at all times. Such membranes include those taught by the Trulson et. al. patent, U.S. Pat. No. 3,977,967, which discloses hollow tubular members having a well defined porosity and a substantially uniform, continuous, adherent, porous coating of preformed, aggregated inorganic metal oxide particles deposited thereon through permeation means. The cohesiveness of membranes of this type is due to relatively weak physical forces, and the cohesive forces between the particles, and the stability of the particulate membrane, would be enhanced by the dehydration of the metal oxide particles. Such dehydration is precluded, however, by the necessity for maintaining the membranes wet at all times to avoid the formation of cracks that would destroy the semipermeable characteristics of the membrane.
Enhanced mechanical and chemical stability are, of course, desired characteristics for inorganic ultrafiltration membranes. In addition, enhanced flexibility would be achieved by the development of ultrafiltration membranes that need not be maintained wet at all times. Thus, the handling, transport and storage of membranes would be facilitated by the elimination of this requirement.
It is an object of the invention, therefore, to provide an improved ultrafiltration membrane.
It is another object of the invention to provide a process for the production of a dry, crack-free, inorganic ultrafiltration membrane.
It is another object of the invention to provide a crack-free, mechanically and chemically stable membrane.
It is a further object of the invention to provide a stable, crack-free, dry ultrafiltration membrane having good perselectivity and flux.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.